A drainage pump has a pump body, a vane, a ring, and a circumferential wall. The pump has a cylindrical small vane pump chamber and a large vane pump chamber extending from and communicating with the small vane pump chamber. A suction inlet communicates with the small vane pump chamber and a discharge outlet communicates with the large vane pump chamber. A chamber slope portion is formed between the small vane pump chamber and the large vane pump chamber. The vane is adapted to be driven by a motor and is rotatably mounted to the pump body. The vane includes a large vane portion received in the large vane pump chamber and a small vane portion extending from the large vane and received in the small vane pump chamber. The small vane portion and the large vane portion rotate together when driven by the motor. A vane slope portion is formed between the large vane portion and the small vane portion. The ring has a central opening, positioned between the large vane portion and the small vane portion. The small vane portion extends through the central opening. The circumferential wall surrounds the periphery of the large vane portion and extends upwardly from the ring. The ring covers the lower end of the large vane portion. The circumferential wall collides against any drainage water returning from the discharge outlet into the pump body to absorb shock and prevent the drainage water from returning directly to the pump body from the discharge outlet.